1. Field of the Invention
This invention relates to the emulation of pipe organs in an electronic musical instrument, and in particular to the emulation of pipe voice changes due primarily to wind pressure variations and acoustic coupling among speaking pipes.
2. Related Art
In order for an electronic musical instrument to emulate the sound of a pipe organ in a manner which is aesthetically pleasing and convincing to the listener, it is desirable to consider factors other than accurate reproduction of the sounds of individual pipes. Although a pipe organ is composed of many pipes which are essentially independent sound sources, there are certain parameters which affect all pipes, or groups of pipes. Further, because pipes are acoustic oscillators, they are subject to influence from the sound waves produced by neighboring pipes.
Pipes in an organ are grouped upon wooden air chambers called windchests. The latter are filled with air under pressure usually generated by an electric centrifugal blower, although other means may be used for supplying the wind used by the organ, including hand-pumped bellows. Between the blower and the windchest, some type of wind regulating device is typically inserted. This device is normally composed of a bellows coupled to an air valve in such a way that the inflation of the bellows causes the air supply into the bellows to be proportionally decreased. The bellows is fitted with weights or springs which resist its inflation such that the device will reach an equilibrium point at which the amount of higher-pressure air introduced into the bellows through the valve is exactly balanced by the air outflow to the pipes. At this equilibrium point, the air pressure inside the bellows (and thus to the pipes) is practically constant, and determined by the amount of weight or spring force resisting the bellows inflation. An increase in demand as more pipes are played causes the bellows to deflate somewhat, thus opening the valve to admit more air. Again, an equilibrium point is reached which balances inflow and outflow.
It is significant that such a regulating device is not perfect. The mechanical gain of the system can be fairly low, producing an output pressure which may tend to drop as more outflow is required. Further, the elasticity of air and the mass of the moving parts of the bellows and valve produce dynamic instability which can appear as a slow response to changes in wind demand, or as overshoot and oscillation if the system damping is low. If the regulator is not in close proximity to the windchest, or if a single regulator serves several windchests, suitable conductors for the pressurized air must be used. These can produce further instability, since they act as a resonant column for the air inside. Pressure loss due to friction may also occur as the air velocity increases in such a conductor.
Although pipe organ builders usually try to design for the best possible regulation of wind, there is always some error, and this error produces subtle changes in pipe pitch and speech. A certain amount of wind "flexibility" is inherent in almost all pipe organs, and actually contributes to the aural signature of the instrument. The extent to which the variations in wind pressure are obvious to the listener varies with the style and heritage of the organ and in some cases introduces desirable nuances into the playing of certain types of organ literature.
A second factor which affects pipe speech when large numbers of pipes are played simultaneously involves the acoustic coupling among neighboring pipes. Since an organ pipe is an acoustic oscillator, it may be influenced by externally-generated sound waves which have frequency components close to those produced by the pipe. This effect is well known by organ tuners as "drawing," and makes the tuning of some pipes difficult because of their tendency to lock on to a nearby pitch. When many pipes of different pitches are played together, a very complex set of fundamental and harmonic frequencies is generated. Some of these frequencies are very close together (within a few Hertz), particularly due to natural harmonics of some notes being close to tempered fundamentals or harmonics of other notes. The audible result of this extremely complex interaction is a gradual detuning of the organ as more pipes are played. This detuning is in addition to the detuning caused by the aforementioned variations in wind pressure.
Although one might expect this detuning to be unpleasant to the human ear, quite the reverse is true. When the human ear is presented with a great number of pitches, as when many pipes are played on a pipe organ, the ear tends to average pitches which are near each other into a single pitch. If it were not for the detuning phenomena described above, a pipe organ would in fact lose some of its aesthetic appeal as more pipes are played, since the multitude of pitches which make up the organ ensemble would no longer be discernable as such. Likewise, electronic organs attempting to recreate the effect of a full pipe organ ensemble will suffer to a degree if sufficient detuning is not provided when many voices are played together. Builders of such instruments have typically resorted to providing a fixed amount of detuning which is always present. Although this improves the full ensemble, it can result in excessive and unpleasant out-of-tuneness when only a few voices are played.
In summary, two factors have been discussed which affect the speech of organ pipes. The first involves the effect of variations in pressure of the wind applied to the pipes, said variations being caused by imperfect operation of wind pressure regulators, and by resonance and friction in the wind conductors. The second factor involves the acoustic coupling among neighboring pipes, causing the pitch of the pipes to be altered as more pipes are played simultaneously.